Components.utils.forceGC
From MDC
This article covers features introduced in Firefox 3
This method allows scripts to force a garbage collection cycle. The Mozilla JavaScript engine will perform garbage collection automatically when the JavaScript heap grows beyond a certain size. This mechanism doesn't account for any native (C++) XPCOM objects hanging off JS objects though. In many cases a JavaScript application will have internal knowledge of JavaScript objects referencing large (trees of) XPCOM objects and know when they are no longer reachable. In this case it can be important to be able to force a garbage collection cycle from JavaScript.
Another instance where it can be useful to force garbage collection is before cleaning out dead reference from a hash of weak JavaScript references.
Users should note that garbage collection is a very expensive operation and should only be forced when it is known to be absolutely necessary.